Electrical energy generation and transmission is a well-known and well-established industry. For many decades, power plants have used turbine generators to produce electrical energy. These power plants commonly operate on non-renewable fuels such as coal and natural gas, or on nuclear fuels, which are typically used to produce steam for turning the turbine generators. Simple cycle and combined cycle natural gas plants utilizing combustion gas turbines are also now playing a larger role in producing electrical energy due to lower natural gas prices and efforts to limit greenhouse gases. Furthermore, combustion turbine-generators comprising a gas turbine engine and a generator are being increasingly used to supply electrical energy during peak loading periods. These combustion turbine-generators may be fueled with, for example, natural gas. Combustion turbine-generators may also be combined with a heat recovery steam generator (HRSG) to form a natural gas combined cycle plant, as would be understood by one of skill in the art.
More recently, there has been an increased focus on the production of electrical energy using renewable energy sources, such as wind power, wave power and solar energy. To this end, large-scale solar power and wind power farms have been built and are now in operation. While solar cells can convert light energy directly into electrical energy, utilizing wind power normally requires the use of a specialized turbine generator or some other type of rotational generator device.
Whether produced by a traditional or renewable energy power plant, generated high-voltage electrical energy is normally transported via transmission lines to substations for distribution to end users. As the process of traditional electrical energy generation has been advanced and refined over many years, so too has the process of transporting electrical energy from a location of generation to a location of use. Furthermore, the rapid growth of the renewable energy industry, especially wind energy, has required the development and installation of new electrical energy transmission infrastructure. As with known and existing electrical energy transmission infrastructure, this new renewable energy infrastructure is utilized to transfer electrical energy from the locations of often remote wind (and other) generation sources to typically heavily populated areas of use.
Because of the long distances over which generated electrical energy must often be transported, a more advanced type of transmission line—known as a series-compensated transmission line—is now frequently employed. As would be understood by one of skill in the art, a series-compensated transmission line differs from a typical transmission line at least due to the inclusion of a plurality of capacitors that are placed in series with the transmission line. Typically, these capacitors are arranged in a bank, or banks, to which a high-voltage transmission line is connected. The capacitors themselves may be arranged in series and/or parallel.
Among other benefits, a series-compensated transmission line provides expanded energy transfer capability in comparison to an uncompensated transmission line at the same voltage level. Consequently, series-compensated transmission lines provide a more economical solution to the long distance transmission of electrical energy.
While series-compensated transmission lines offer an advantage over traditional (non-compensated) transmission lines, as described above, it has been discovered that the series capacitor banks of series-compensated transmission lines, coupled with transmission line reactance, can create natural resonant frequencies on the associated transmission networks. It has also been determined that if the natural resonant frequency of a transmission network is the synchronous frequency complement of any of the natural mechanical frequencies of the spring-mass system of a connected turbine-generator, the turbine system may experience subsynchronous resonance (SSR) that can damage the turbine-generator shaft.
There are several known ways in which SSR can negatively affect a turbine generator system. For example, Torsional Interaction (TI), Induction Generator Effect (IGE), and Torque Amplification (AI) are all known effects of SSR on traditional fossil fuel generators. See, e.g., P. M. Anderson, and R. G. Farmer, Series Compensation of Power Systems, ISBN 1-888747-01-3.
SSR can also negatively affect renewable electrical energy generation devices. In fact, the use of series-compensated transmission lines in conjunction with renewable electrical energy generation devices has revealed a new type of power system subsynchronous interaction. More specifically, this new type of power system subsynchronous interaction occurs between series-compensated transmission lines and wind turbines—particularly, Type-3 wind turbine-generators (WTG). Type-3 WTGs typically have a wind turbine shaft that is coupled via a gearbox to a Double Fed Induction Generator (DFIG). A stator portion of the DFIG is normally connected directly to the power grid, while a rotor portion of the DFIG is connected to the power grid through a power converter system based on power electronics. A crowbar circuit is commonly also present to prevent an overvoltage or overcurrent condition of the DFIG from damaging the power converter system. In the event of such a condition, the rotor side output of the DFIG is disconnected from the power converter system and connected to the crowbar circuit until the fault condition is cleared.
One particular and real-world example of such a wind turbine power system subsynchronous interaction occurred within the Electric Reliability Council of Texas (ERCOT) region in October 2009. In this case, a subsynchronous interaction occurred because the control electronics of a Type-3 WTG reacted so quickly to a detected system disturbance that modes of oscillation were created in the associated series-compensated transmission networks. More specifically, the ERCOT event was triggered by a fault on one of two series-compensated 345 kV transmission lines that export the power generated by two wind farms. The protection system control electronics correctly tripped the faulted line according to design to isolate the fault within 3 cycles. However, the loss of one transmission line left the two wind farms radially connected to the other series-compensated transmission line and, consequently, the oscillation began.
FIGS. 1A-1B graphically depict the line currents and bus voltages, respectively, as recorded by a digital relay at the interconnection point of the wind farm and the transmission grid during the ERCOT event. A dedicated digital fault recorder at the series capacitor bank site also recorded the series capacitor current through the complete sequence of the event. The recorded series capacitor current is shown in FIG. 2.
As can be observed, after about 200 milliseconds into the ERCOT oscillation event, the bus voltage at the interconnection point reached 1.45 per unit, and the current magnitude of the combined, two wind-farm output, increased to about 9 times that of the pre-fault current magnitude. It is at this point that damage to the wind turbine crowbar circuit was estimated to have started.
During the entirety of the oscillation event—which lasted for only about 1.5 seconds—the only protection system reaction that occurred was the closing of a series capacitor bank bypass breaker at the instruction of the capacitor bank control system when the Metal Oxide Varistors (MOVs) thereof started conducting excessively. The MOV conducting current during the course of the oscillation event is graphically shown in FIG. 3.
The spectrum analysis of the recorded line current and bus voltage after the ERCOT event line fault reveals significant subsynchronous oscillation (SSO). FIGS. 4A-4B graphically represent the spectrum of the phase currents and voltages, respectively, that occurred during the ERCOT oscillation event. It may be observed that the spectrum plots of all three phases reveal a very similar spectrum signature, and the frequency of the subsynchronous component is approximately 24 Hz. The spectrum of the line currents further reveals that the magnitude of the subsynchronous component is 250% larger than the fundamental frequency component, while the magnitude of the bus voltage subsynchronous component is about 30% of the fundamental frequency component. That is, the subsynchronous component within the current signal is more obvious than the subsynchronous component within the voltage signal.
The above-described 1.5 second ERCOT subsynchronous oscillation event accompanied by the depicted overvoltage caused damage to several wind generators and had an adverse effect on the fatigue life expenditure of other system equipment.
Subsequent, detailed simulation of the ERCOT (and other) oscillation event data has revealed that the frequency of oscillation in such an event could range from about 5 Hz to 55 Hz in power systems having a fundamental frequency of 60 Hz and about 5 Hz to 45 Hz in power systems having a fundamental frequency of 50 Hz. If the subsynchronous resonance is caused by interaction between the series capacitor banks and the renewable generation control system, the oscillation is referred to as Subsynchronous Control Interaction (SSCI).
Interestingly, SSCI problems are not necessarily limited to renewable electrical energy generation systems. Rather, SSCI can potentially cause damage to the shaft system of traditional steam turbines, and in extreme cases, to the shaft system of combustion gas turbines as well. For example, if one of the resonance frequencies of the series compensated transmission network is close to the complementary mechanical system frequency of the shaft of a traditional steam turbine generator connected to the same power grid, then the two oscillatory systems can interact with each other. In the same operating conditions, the interaction can result in damaging shaft torques on a steam turbine generator shaft.
For entities (e.g., utilities) that may encounter subsynchronous resonance (SSR) problems, protective relays have become an important component of the overall strategy of protection. For example, a known torsional stress relay (TSR), which is based on shaft speed measurement, was developed years ago and has been widely applied to protect generator shafts. However, known relays, such as but not limited to the TSR, cannot address the unique challenges associated with SSCI.
SSCI also poses challenges to even modern digital relays, because the signal processing algorithms of such relays are typically designed to extract fundamental frequency signals and to filter out non-fundamental frequency signals. In this regard, see, e.g., L. G. Gross, Sub-synchronous Grid Conditions: New Event, New Problem, and New Solutions, 37th Annual Western Protective Relay Conference, Spokane, Wash., October, 2010.
Efforts have been made to reconstruct the sub-harmonic operational quantities frequently associated with an electrical energy generation system by compensating the attenuation of digital filters that were originally designed to extract the fundamental frequency. See, e.g., E. O. Schweitzer and D. Hou, Filtering for Protective Relays, 19th Annual Western Protective Relay Conference, Spokane, Wash., October, 2010. While such a method can utilize a traditional relay platform, its effectiveness is nonetheless questionable because of limitations associated with signal processing.
Consequently, it can be understood that it would be beneficial to provide a mechanism by which to detect and react to such SSCI oscillation events with sufficient speed and in a manner that prevents damage to system components. Exemplary device, system and method embodiments described herein provide for such mechanisms.